A CCTV DVR is the digital video recorder part of the CCTV System. It is the part that records and manipulates the CCTV footage.
The analogue video information is fed into the DVR and converted to digital signals. The DVR processes these and converts the information for use by the CCTV controller.
With a DVR you can then record, split, pause or save the video information to help your CCTV controller make the most of the information.